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Dietary studies

Dietary studies that look at differences in peoples' diets and how these appear to affect their health are known as epidemiological studies. Many epidemiological studies have shown that diets rich in cruciferous vegetables reduce the risk of developing chronic diseases including some forms of cancer and heart disease.

Epidemiological studies don’t, on their own, show that lifestyle factors such as cruciferous vegetables in the diet cause a reduction in chronic disease. Instead they highlight general trends in the population. Scientists can follow up trends identified in epidemiological studies with experiments that uncover exactly how, in the human body, different foods can have beneficial effects. A plausible mechanism, coupled with strong epidemiological data, provides the best evidence for health effects related to diet. This quality of evidence is needed to support health claims and policies underlying governmental health, diet and nutritional advice.

There are different general types of epidemiological study. One type looks at a group of people with a certain health condition, for example cancer. The diets of these people are then compared with a carefully selected ‘control’ group of people who haven’t developed cancer. The studies, known as retrospective case-controlled studies, can associate dietary differences with the risk of getting health conditions, but they suffer from a number of problems. Firstly, they rely on people accurately recalling their dietary habits, usually through filling in questionnaires, which can be a challenge. Also, selecting the healthy ‘control’ group of people is incredibly hard. Ideally the groups should be very similar, but a problem with measuring the effects of a healthy diet is that eating plenty of fruit and vegetables is often part of a generally healthier lifestyle, and is affected by socioeconomic factors. The design of the study and the interpretation of the data can overcome some of these problems.

Some studies have shown that cruciferous vegetables may reduce the risk of prostate cancer[i], bladder cancer[ii], breast cancer[iii]and other conditions, a list of which is available. Some of these studies have tried to improve the study design by measuring compounds in urine instead of reported diet to assess cruciferous vegetable intake.

More recently, ‘cohort studies’ have also been reporting their findings. Cohort studies follow very large groups of people over a period of time, and correlate their diet and lifestyle with the development of diseases and conditions. These studies involve tens of thousands of people and follow them over decades.

Cohort studies have shown some protection against cardiovascular disease from cruciferous vegetables [iv] as well as prostate cancer[v][vi] and others detailed in the table.

In addition, a number of ‘meta-analyses’ have attempted to combine many of the different studies undertaken by different research groups around the world to give an overview.

The strongest evidence for health-protecting effects of eating certain foods comes from Human Intervention Trials, where a group of human volunteers have their diet supplemented with the food being tested and the effects compared with a similar group who do not receive the supplementation. With good study design, neither the volunteers nor the researchers should be able to tell which diets are being supplemented, avoiding any bias in the study.

The development of Beneforté broccoli is giving researchers the ability to give volunteers dietary interventions containing different levels of glucoraphanin. The first of these trials[ix] showed that high glucoraphanin broccoli (Beneforté) was able to ‘re-tune’ metabolic processes in our cells linked to health outcomes. In further studies, published in 2015, we have shown that a diet rich in high glucoraphanin broccoli reduced blood LDL cholesterol levels. Two independent dietary intervention trials showed that people eating 400g of high glucoraphanin broccoli saw an average reduction of 6% in the LDL cholesterol after 12 weeks.[x]